Integrated filter circuit with variable frequency characteristics

ABSTRACT

A filter integrated circuit having a reference level generator circuit for attenuating its input signal and generating a reference level signal, a pseudofilter circuit including resistors and a variable capacitor as filter elements, and an error amplifier unit for comparing the level of the output signal of the reference level generator circuit with the level of the output signal of the pseudofilter circuit and for generating an automatic adjustment control signal in accordance with the level difference between the output signals. The automatic adjustment control signal is supplied to the pseudofilter circuit to change the capacitance value of the variable capacitor in the pseudofilter circuit so that the output signal level of the pseudofilter circuit may become equal to the level of the reference signal. The automatic adjustment control signal is also supplied to at least one filter circuit to change the capacitance of a variable capacitor included therein. As a result, the deviation in the filter characteristics caused by deviations in the resistance of a resistor and in the capacitance of the capacitor is corrected.

BACKGROUND OF THE INVENTION

The present invention relates to a filter integrated circuit suitablefor integration of a filter into a monolithic IC formed on a siliconwafer or the like.

As lowpass, highpass and bandpass filters or phase equalizers forproducing desired signals in conventional electronic circuits, blockfilters having discrete components of inductance L, capacitance C andresistance R have been widely used. As more and more electronic circuitsare realized as integrated circuits (monolithic IC's), these filtersbecome obstacles to cost reduction and reduction of size and weight ofelectronic circuits. Especially for portable devices in which mobilityis an important factor, reduction in size and weight is important and itis demanded to realize filters in the form of integrated circuits.

Since it is difficult to realize the inductance L as an integratedcircuit, filters suitable for integration are active filters which canbe formed by using only the capacitance C and resistance R. For example,twin-T-shaped trap filters can be formed by using only capacitors andresistors. The trap frequency fr of such a filter is represented as

    fr=1/2πCaRa

where Ca is the capacitance of the capacitor used in the twin-T-shapedtrap filter and Ra is the resistance of the resistor used in thatfilter.

When such a trap filter is to be integrated, deviations in thecapacitance value and the resistance value pose a problem. That is tosay, the values of the capacitance and resistance in the IC are affectedby deviations in the impurity concentrations and mask alignment etc. Forexample, the absolute value of the capacitance varies by ±10 to 15% andthe absolute value of the resistance varies by ±10%. These deviationsare rather large values. In the above described example, therefore, thetrap frequency of the integrated twin-T-shaped trap filter also variesby ±20 to 25% in the worst case, the practical use being extremelydifficult. In accordance with a countermeasure disclosed in JapanesePatent Publication No. 58083/82, the resistance value of the resistorlocated on the IC chip is changed by using the laser trimming to correctthe deviation. Although this countermeasure has been used, many problemsstill remain with respect ot the precision and the yield rate.

In variable attenuation circuits disclosed in Japanese PatentPublication No. 58083/82 and U.S. Pat. No. 3,761,741, the fact that theemitter resistance of the transistor is varied by a change in the DCcurrent is used. It is known that the variation in the filtercharacteristics caused by deviations of element values of the IC can becorrected by using the similar technique. However, it is difficult toapply this technique to all filters including trap filters. In addition,deviations of elements of the IC must be corrected by externaladjustment, resulting in a higher cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a filter integratedcircuit which is capable of automatically correcting the variation inthe filter characteristics caused by deviations in the integratedcapacitance and the integrated resistance without requiring any externaladjustment ot ensure the predetermined performance and which is freefrom the above described problems of the prior art.

A filter integrated circuit in accordance with the present inventioncomprises a reference level generator circuit for receiving an inputsignal having a reference frequency and for generating a reference levelsignal, a pseudofilter circuit filtering said input signal andcomprising an integrated resistor and an integrated capacitor having avariable capacitance value, an error amplifier unit for receiving theoutput signal of said reference level generator and the output signal ofsaid pseudofilter circuit and for generating an automatic adjustmentcontrol sinal in accordance with the level difference between thoseoutput signals, means for supplying the automatic adjustment controlsignal to the pseudofilter circuit and for thereby changing thecapacitance value of the variable capacitor so that the level of theoutput signal of the pseudofilter circuit may become equal to the levelof the reference level signal, and a filter circuit including anintegrated resistor and an integrated variable capacitor whichrespectively have high ratio precision with respect to the correspondingelements of the pseudofilter circuit, i.e., the resistor and thecapacitor of the pseudofilter circuit. The automatic adjustment controlsignal is supplied to the filter circuit to change the capacitance valueof the variable capacitor thereof so as to correct the deviation in thefilter characteristics.

Here, the ratio precision refers to the precision of a ratio betweenvalues of circuit components. For example, when the ratio betweenresistance values of two resistors remains unchanged even if individualresistance values deviated fromj their nominal values, it is said thatthe ratio precision is high. This holds true also for capacitancevalues. In a semiconductor integrated circuit, high ratio precisionbetween circuit component values on one chip can be obtained althoughindividual values deviate from their nominal values.

In a filter integrated circuit in accordance with the present invention,the automatic adjustment control signal is generated to make the outputsignal level of the pseudofilter circuit equal to the reference value.That is to say, the automatic adjustment control signal corrects thedeviation in the filter characteristics of the pseudofilter circuitcaused by deviations in the element values and the automatic adjustmentcontrol signal is concurrently supplied to the filter circuit. The ratioprecision between an element value of the filter circuit and thecorresponding element value of the pseudofilter circuit is high. If thefilter characteristics of the pseudofilter circuit vary due todeviations in the element values, therefore, the filter characteristicsof the filter circuit vary in the same way as the pseudofilter circuit.Accordingly, it is possible to correct the deviation in thecharacteristics of the filter circuit by using the automatic adjustmentcontrol signal which is used to correct the deviation in thecharacteristics of the pseudofilter circuit.

In the filter integrated circuit, the reference level generator circuitcan be formed by a plurality of integrated resistors or externalresistors having high ratio precision so as to attenuate the referenceinput signal by using those resistors to produce the reference levelsignal. The error amplifier unit can be composed of a detector circuitfor detecting the output signal of the reference level generatorcircuit, another detector circuit for detecting the output signal of thepseudofilter circuit, and an amplifier for receiving the output signalsof both detector circuits, amplifying the level difference between thoseoutput signals, and generating the automatic adjustment control signal.The frequency of the input signal to the filter integrated circuit maybe variable. In this case, automatic adjustment is carried out forsignals of respective frequencies.

In accordance with the present invention, the deviation in thecharacteristics of a filter comprising an integrated resistor and anintegrated variable capacitor as elements contained in a semiconductorintegrated circuit is automatically adjusted. And it is possible toimprove the precision of the filter and eliminate the adjustment whichhas been heretofore conducted filter by filter. In accordance with thepresent invention, therefore, a large-sized block filter which hasheretofore been used as an external part can be integrated withoutrequiring adjustment. As a result, it becomes possible to reduce thecost of circuits in the filter unit as well as the size, weight and thenumber of parts of the unit.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome apparent by reference to the following description andaccompanying drawings wherein:

FIG. 1 is a circuit diagram of a twin-T-shaped trap filter;

FIG. 2 is a characteristic diagram for illustrating the deviation in thefrequency characteristics of the filter illustrated in FIG. 1;

FIG. 3 is a block diagram for illustrating the configuration of anembodiment of a filter integrated circuit according to the presentinvention;

FIG. 4 shows a concrete example of a reference level generating circuitused in the filter integrated circuit of FIG. 3;

FIG. 5 shows a concrete example of a pseudofilter circuit used in thefilter integrated circuit;

FIG. 6 shows waveforms of signals appearing at various points of thefilter integrated circuit;

FIG. 7 is a characteristic diagram of a varactor diode used as avariable capacitor in the pseudofilter circuit;

FIG. 8 shows deviations in the frequency characteristics of thepseudofilter circuit;

FIG. 9 shows a concrete example of a filter circuit used in theintegrated circuit;

FIG. 10 shows another concrete example of a filter circuit used in thefilter integrated circuit;

FIG. 11 is a circuit diagram of another embodiment of a filterintegrated circuit according to the present invention; and

FIG. 12 is a circuit diagram of still another embodiment of a filterintegrated circuit according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Prior to description of an embodiment of the present invention, theconfiguration of a twin-T-shaped trap filter suitable to integrationwill now be described. FIG. 1 shows the configuration of a well-knowntwin-T-shaped trap filter. If resistance and capacitance values of FIG.1 are chosen as

    R.sub.1 =R.sub.2 =2R.sub.3 =Ra

    C.sub.1 =C.sub.2 =C.sub.3 /2=Ca,

it follows that fr=1/2πCaRa. In FIG. 1, v_(i) is an input signal andv_(o) is an output signal.

When a trap filter is intergrated, the trap frequency is deviated asresult of deviations in the capacitance and resistance values of theintegrated circuit. In the integrated circuit, the absolute value of thecapacitance may vary as much as ±10 to 15% and the absolute value of theresistance may vary as much as ±10% . In this case, the trap frequencyfr of the trap filter illustrated in FIG. 1 varies as much as ±20 to 25%in the worst case. FIG. 2 shows that the trap frequency varies in rangeextending from a to b as a result of variation in the resistance andcapacitance values of the filter circuit illustrated in FIG. 1. Thepresent invention provides a filter integrated circuit having a functionof automatically correcting such variation of the trap frequency of thetrap filter.

Embodiments of the present invention will now be described by referringto drawings. FIG. 3 is a block diagram for illustrating theconfiguration of an embodiment of a filter circuit according to thepresent invention. In FIG. 3, a signal 3 having a constant referencefrequency fin is applied to an IC pin 2 of an IC 1. The input signal 3is applied to a reference level generator circuit 4 and a pseudofiltercircuit 5 in the IC 1. The reference level generator circuit 4attenuates the input signal to produce a reference level signal. Thereference level signal is detected in a detector circuit and thedetected output is fed to one input terminal of an error amplifier 7.The pseudofilter circuit 5 is a filter circuit comprising a resistor anda variable capacitor (provided by a varactor) as elements. Thepseudofilter circuit 5 filters the input signal and supplies thefiltered signal to a detector circuit 8. The detector circuit 8 detectsthe signal and supplies the detected signal to the other input terminalof the error amplifier 7. The error amplifier 7 amplifies the leveldifference between the output of the detector circuit 6 and the outputof the detector circuit 8. The resultant voltage signal is supplied tothe varactor diode in the pseudofilter circuit 5 via a lead 9. In thepseudofilter circuit 5, the voltage signal supplied from the erroramplifier 7 changes the capacitance value of the varactor diode tochange the filter characteristics so that the level of the filteredsignal level will be equal to that of the reference signal levelsupplied from the reference level generator circuit 4. The referencelevel generator circuit 4, the pseudofilter circuit 5, the detectorcircuits 6 and 8, and the error amplifier 7 constitute an automaticadjustment control signal generator circuit 10. The automatic adjustmentcontrol voltage signal supplied from the error amplifier 7 adjusts andcorrects the deviation in the filter characteristics caused bydeviations in the resistance and capacitance of the pseudofilter circuit5. The automatic adjustment control voltage signal is supplied to filtercircuits 11 and 12 provided in the IC 1. The circuits 11 and 12 includeresistors varactor diodes as their components. The resistor and varactordiode in each of filter circuits 11 and 12 are formed in high ratioprecision with respect to the resistor and varactor diode of thepseudofilter circuit, respectively. In each of filter circuits 11 and12, the automatic adjustment control voltage signal changes thecapacitance value of the varactor diode to adjust the deviation in thefilter characteristics caused by deviations in the resistance andcapacitance values. The input and output of the filter circuit 11 arerespectively connected to IC pins 13 and 14 so as to be used outside theIC 1. The filter circuit 12 is used by the circuit within the IC 1.

The embodiment of FIG. 3 will now be described in more detail. FIG. 4shows an example of a conrete circuit of the reference level generatorcircuit 4 and FIG. 5 shows an example of a concrete circuit of thepseudofilter circuit 5. FIG. 6(a) shows a waveform of the input signal2. FIG. 6(b) shows waveforms appearing at the output of the referencelevel generator circuit 4 and the output of the detector circuit asrepresented by the solid line and the broken line, respectively. FIG.6(c) shows waveform appearing at the output of the pseudofilter circuit5 and the output of the detector circuit 8 as represented by the solidline and the broken line, respectively. FIG. 7 shows a characteristicdiagram of the varactor diode included in the pseudofilter circuit 5.FIG. 8 shows frequency characteristics of the pseudofilter circuit 5.FIG. 9 shows a concrete example of the circuit diagram of the filtercircuit 11. FIG. 10 shows a concrete example of the circuit diagram ofthe filter circuit 12.

The reference level generator circuit 4 attenuates the input signal 3 bya constant radio with precision. The reference level generator circuit 4can be formed by integrated resistors 15 and 16 as shown in FIG. 4, forexample. Since the ratio of element values in the IC can be obtainedwith sufficiently high precision, the attenuation between the input andoutput of the reference level generator circuit 4 represented as##EQU1## can also be realized with sufficiently high precision. Thepseudofilter circuit 5 is a filter circuit including an integratedresistor and a varactor diode which serves as an integrated capacitorwhich has a capacitance value varied by the voltage applied thereto. Forexample, the pseudofilter circuit 5 can be formed by integratedresistors 17 and 18, a varactor diode 19, and a constant voltage source20 as shown in FIG. 5. The resistance value R₁₈ of the integratedresistor 18 and the capacitance value C₁₉ of the varactor diode 19 forma CR filter of the first order. The cutoff frequency f_(c) isrepresented by

    f.sub.c =1/2πR.sub.18 C.sub.19.

The constant voltage source 20 applies the DC voltage to the anode ofthe varactor diode 19 via integrated resistors 17 and 18. On the otherhand, the output voltage of the error amplifier 7 is supplied to thecathode of the varactor diode 19 in the negative feedback form.

The capacitance value of the varactor diode 19 is varied by the voltageapplied across it. When the capacitance of a base-emitter junction isused as the varactor diode 19, the capacitance can be represented as##EQU2## where: Cj=base-emitter junction capacitance

Cj (O)=base-emitter junction capacitance at zero bias

Vj=emitter-base voltage (reverse-biased diode voltage)

φ=built-in voltage

α=voltage-dependent coefficient

K=log [Cj(0)φ.sup.α ]

An example of characteristics of the base-emitter junction capacitanceis shown in FIG. 7. When the power supply voltage is 5 V, Vj may be 0 to3 V and Cj can be varied at least by ±20 to 25% with respect to itstypical value.

The outputs of the reference level generator circuit 4 and thepseudofilter circuit 5 as represented by the solid lines of FIGS. 6(b)and 6(c) respectively are subjected to peak detection in the abovedescribed detector circuits to become signals as represented by brokenlines to FIGS. 6(b) and 6(c), respectively. These output signals of thedetector circuits as represented by broken lines are supplied to theerror amplififer 7. As the automatic filter adjustment control voltagesignal 9 which has been subjected to negative feedback, the output ofthe error amplifier 7 is supplied to one end of the varactor diode 19defining the filtering characteristics of the pseudofilter circuit 5 sothat the outputs of the detector circuits 6 and 8 may equal each other,i.e., levels 21 and 22 respectively as shown in FIGS. 6(b) and 6(c) maybecome equal to each other. By the automatic adjustment control voltagesignal, the capacitance value of the varactor diode 19 is automaticallyvaried to absorb the deviation in the pseudofilter circuit 5.

Each of two filter circuits 11 and 12 comprises an integrated resistorand a variable capacitor to attain the desired filter characteristicsand is supplied with the automatic adjustment control signal 9.

Since elements integrated on one chip can be formed with high ratioprecision, the deviation in the frequency characteristics of thepseudofilter circuit 5 can be made nearly equal to that of each of thefilter circuits 11 and 12. Accordingly, it is possible to automaticallyabsorb deviations in the frequency characteristics of the filtercircuits 11 and 12 by using the automatic adjustment control signal 9.

The operation of the circuit of FIG. 3 will now be described in moredetail. It is now assumed that the integrated resistors 15 and 16 aredefined so that the reference level generator circuit 4 has anattenuation loss of 3 dB. If the sum of deviations of the integratedresistor and the varactor diode is -20%, the pseudofilter circuit 5 hascharacteristics as represented by 23 FIG. 8. If the input signal 3having a frequency fin is then supplied to the circuit 3, the output ofthe pseudofilter circuit 5 becomes larger than that of the referencelevel generator circuit 4. And the outputs of the pseudofilter circuit 5and the reference level generator circuit 4 are fed back to thepseudofilter circuit 5 via the detector circuits 6 and 8 as well as theerror amplifier 7 so as to reduce the voltage applied to the varactordiode of the pseudofilter circuit 5. Since the decrease in the voltageapplied to the varactor diode increases the capacitance value as shownin FIG. 7, the frequency characteristics 23 of FIG. 8 is shifted to theleft to produce the frequency characteristics 24 of FIG. 8. That is tosay, the capacitance value of the variable capacitor is so varied thatthe outputs of the reference level generator circuit 4 and thepseudofilter circuit 5 may become equal to each other at the frequencyfin. Since the capacitance value of the varactor diode varies by ±20 to25% in accordance with Vj, the maximum deviation can be absorbed. If thetotal deviation of the integrated resistor and the varactor diode is themaximum value of +20%, the pseudofilter circuit 5 assumescharacteristics 25 as illustrated in FIG. 8. In this case, it is amatter of course that the pseudofilter circuit 5 has eventually thefrequency characteristics 24 of FIG. 8 upon receiving the fin input.

As described above, the automatic adjustment control signal 9 forautomatically absorbing the deviations of the integrated resistor 18 andthe varactor diode 19 is obtained. Because of existence on the samechip, the integrated resistor 18 and the varactor diode 19 can be formedwith sufficiently high ratio precision with respect to the integratedresistors 30 and 31 and the varactor diodes 32 and 33 illustrated inFIGS. 9 and 10, respectively. Cutoff frequencies f_(c) (11) and f_(c)(12) of the filter circuits 11 and 12 exemplified in FIGS. 9 and 10 arerepresented as ##EQU3## where n₁ to n₄ are constants. Thus, deviationsin the filter circuits 11 and 12 can be automatically absorbed,resulting in integrated filters requiring no adjustments.

Further, since the reference of the negative feedback is the attenuation##EQU4## of the reference level generator circuit 4 as described above,the filter characteristics depend upon neither the temperature, nor thepower supply voltage, nor the level change of the input signal 3. As aresult, desired stable filter characteristics can always be attained.

In the embodiments described above, the input signal 3 is suppliedoutside the IC 1. However, it is a matter of course that the similareffect can be obtained even if the input signal 3 is supplied from acircuit contained in the same IC. In the video tape recorder, forexample, a conrete signal source is the chrominance subcarrier (3.58 MHzin NTSC) generated with high precision in the color signal processingcircuit by using a crystal oscillator.

In the above description, the detector circuits 6 and 7 send out thehalf-wave rectified waveforms. If full-wave rectified waveforms aresubjected to peak detection, more stabel filter circuits can berealized.

FIG. 11 shows another embodiment of the present invention. Referencenumerals that are like reference numerals in FIG. 3 refer to likecomponents. Constant voltage sources 40, 41 and 42 supply the samevoltage. This voltage minus the base-emitter voltage of a transistorwhich is approximately 0.7 V is supplied to anode sides of the varactordiodes 19, 43, 44 and 45 and also supplied to the detector circuits 6and 8. Reference numerals 46 to 48 denote integrated capacitors.Reference numerals 47 to 52 and 53 to 61 denote npn transistors andintegrated resistors, respectively.

A filter circuit 11 is a twin-T-shaped trap filter described before. Ifelements are selected as

    R.sub.58 =R.sub.59 2R.sub.60 =R.sub.b

    C.sub.44 =C.sub.45 =C.sub.43 /2=C.sub.b

then the trap frequency fr can be represented as

    fr=1/2πR.sub.b C.sub.b.

Since the elements on one chip can be realize with high ratio precision,R_(b) and C_(b) can be represented as

    R.sub.b =n.sub.5 R.sub.18

and

    C.sub.b =n.sub.6 C.sub.19

where n₅ and n₆ are constants. Thus, it allows that

    fr=1/2πn.sub.5 n.sub.6 R.sub.18 C.sub.19.

By the automatic adjustment of the pseudofilter circuit 5, the componentdeviations in the twin-T-shaped circuit can also be absorbed withoutrequiring any adjustment.

FIG. 12 shows still another embodiment of the present invention.Reference numerals that are like reference numerals in FIGS. 3 and 11refer to like components. Varactor diodes 62 and 63, npn transistors 64to 67, constant current sources 68 and 69, integrated resistors 70 to77, a constant voltage source 78, and AC signal bypassing capacitor 79,and an IC pin 80 are shown in FIG. 12. A resistor 74 having a highresistance value is used to supply bias voltage for passing only the DCvoltage signal. Since the resistor 74 has a high resistance value, an ACsignal flowing through the resistor 74 is largely attenuated as comparedwith the AC signal flowing to the varactor diodes 62 through thecapacitance 79 and is neglizible. The automatic adjustment controlsignal from the error amplifier circuit 7 is supplied to the varactordiode 62 through the resistor 74. Reference numeral 81 denotes adifferential amplifier. The differential amplifier 81 constitutes apositive feedback type lowpass filter of the second order in conjunctionwith resistors 72 and 73 as well as varactor diodes 62 and 63. Theeffects of the present invention described before are obtained in thisembodiment as well.

In the embodiment of FIG. 12, the filter circuit forms a lowpass filter.As an alternative, a highpass filter or a bandpass filter may be used.In this case, the pseudofilter circuit is formed similarly to FIG. 12.And the capacitance value of the varactor diodes included in the filtercircuit comprising the highpass filter or the bandpass filter is changedby the automatic adjustment control signal so as to correct thedeviation in the filter characteristics.

In the above described embodiments, the input signal has a constantfrequency. However, an input signal having a variable frequency may alsobe used.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from the presentinvention in the broader aspect.

We claim:
 1. A filter integrated circuit including at least a resistorand a capacitor comprising:a reference level generator circuit forattenuating an input signal and generating a reference level signal; apseudofilter circuit for filtering said input signal, said pseudofiltercircuit including a resistor and a variable capacitor as filterelements; a detector circuit for detecting the output signal of saidreference level generator circuit; another detector circuit fordetecting the output signal of said pseudofilter circuit; an erroramplifier for receiving the output signals of said detector circuits,comparing signal levels of both output signals, and producing anautomatic adjustment control signal in accordance with the differencebetween said signal level; means for supplying said automatic adjustmentcontrol signal from said error amplifier to said pseudofilter circuitfor changing the cpacitance value of said variable capacitor to reducesaid signal level difference; and a filter circuit including a resistorand a variable capacitor formed with high ratio precision with respectto filter elements of said peudofilter circuit, wherein said filtercircuit is also coupled to said means for supplying said automaticadjustment control signal so that the cpacitance value of said variablecapacitor of said filter circuit is changed by said automatic adjustmentcontrol signal to change the filter characteristics of said filtercircuit.
 2. A filter integrated circuit according to claim 1, whereinsaid variable capacitors of said pseudofilter circuit and said filtercircuit are comprised of varactor diodes.
 3. A filter integrated circuitaccording to claim 1, wherein said reference level generator circuitcomprises resistors, said pseudofilter circuit comprises resistors andvariable capacitors, and said filter circuit comprises resistors andvariable capacitors, said respective circuits being formed on the sameintegrated circuit substrate.
 4. A filter integrated circuit accordingto claim 1, wherein said reference level generator circuit, saidpseudofilter circuit, and said filter circuit further comprisestransistors as active components, respectively, said respective circuitsbeing formed on the same integrated circuit substrate.